In a communication system, a terminal connects with one of cells to perform communication. The “cell” is determined by an area covered by each base station, that is, a area reached by a signal transmitted from each base station, or one of divided areas (that is, so-called sectors) into which the area is divided, and a frequency. Each cell has a unique cell identification information (that is, a cell ID) associated thereto. The terminal is given the identification information of a cell with which the terminal connects.
When the reception power of a signal transmitted reduces in the cell with which the terminal connects, due to move of the terminal, normally, handover to a cell in which the reception power is higher is executed. However, if the terminal enters a “coverage hole”, there might be cases where communication between the terminal and the cell with which the terminal connects is disconnected before handover is executed. The “coverage hole” means an area which a radio wave from a base station does not reach at all, or an area where the reception power rapidly falls, as compared to the surrounding areas. Disconnection of communication caused by such a rapid fall of the reception power is sometimes called radio link failure (RLF).
When an RLF occurs, the terminal or the base station remains in a state where setting information and the like for a service performed immediately before the occurrence of the RLF is maintained, that is, a RRC connection state where connection is maintained using the radio resource control (RRC). When the radio wave condition is not improved for a predetermined period of time after the detection of an RLF, the state of the terminal and the base station transitions to an idle state. The idle state means a state where the terminal waits for the generation of new data or a call from the base station, and no data exchange takes place. In the idle state, unlike the RRC connection state, setting information and the like for a service previously performed is not maintained.
After the transition to the idle state, the terminal gives a “request for reconnection” to the base station. That is, a reconnection sequence is executed. Thus, the terminal is enabled to continue communication with the base station using a cell which has been reconnected.
In the Third Generation Partnership Project (3GPP) as an international standardization organization, LTE-Advanced (Release 10) which has largely extended functions has been already developed. When a LTE-Advanced compatible terminal detects an RLF, the terminal generates a report (an RLF-Report) including identification information of a cell with which the terminal currently connects and the positional information of the terminal itself and holds it. Then, when the terminal receives, after reconnecting with a cell, a terminal information request (a UE information Request) transmitted from the base station, the terminal transmits, to the base station, a terminal information response (UE information Response) including the RLF-Report held by the terminal, in response to the terminal information request. Then, when a base station of a cell with which the terminal connects is different between before and after reconnection, the base station of the cell after reconnection notifies the base station of the cell before reconnection, that is, the cell where the RLF occurred, of the RLF-Report using RLF INDICATION.
Japanese Laid-open Patent Publication No. 2012-90267 describes the related technique.